Method and apparatus for flue gas desulfurization

ABSTRACT

A method for flue gas desulfurization in which sulfur oxides in the flue gas are converted into powdery ammonium compound, including providing an aqueous ammonia spraying device configured to atomize aqueous ammonia into droplets, cooling flue gas containing sulfur oxides down to a temperature between a saturation temperature of water and 80° C., adjusting the aqueous ammonia spraying device such that the aqueous ammonia is atomized into droplets having a Sauter mean diameter of 0.5 μm to 30 μm, and spraying the aqueous ammonia into the flue gas which has been cooled in the cooling step.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and an apparatus for flue gasdesulfurization, and more particularly to a method and an apparatus forflue gas desulfurization by converting sulfur oxides typicallycomprising sulfur dioxide into powdery ammonium compound and collectingthe ammonium compound.

2. Discussion of the Background

In a method for flue gas desulfurization, aqueous ammonia is sprayed andinjected into flue gas containing sulfur oxides (SOx) such as combustionflue gas emitted from a boiler, and aqueous ammonia reacts with sulfuroxides to produce ammonium compound. Particularly, sulfur dioxide (SO₂)which is a main component of SOx reacts with aqueous ammonia(m·NH₃+n·H₂O) and oxygen (O₂) to produce ammonium sulfate ((NH₄)₂SO₄) asby-product. This chemical reaction is expressed in the following formula(1).

SO₂+2NH₃+H₂O+1/2O₂→(NH₄)₂SO₄+437.7 kJ/mol   (1)

As typically indicated in the above formula (1), the reaction in whichaqueous ammonia reacts with sulfur oxides to produce ammonium compoundis exothermic reaction, and water contained in aqueous ammonia sprayedand injected is consumed by reaction as shown in the left side of theformula (1) and is evaporated while removing the heat of the reaction.In general, the desulfurizing reaction as shown in the above formula (1)is liable to proceed as the temperature of flue gas is lowered, andhence removal of the heat caused by vaporization of water contained inaqueous ammonia prevents the temperature of flue gas from increasing dueto the heat of the reaction and therefore prevents the desulfurizingreaction from not proceeding. Therefore, if water sprayed and injectedas aqueous ammonia is controlled to a certain amount which is the sum ofthe amount consumed by reaction and the amount required for preventingthe temperature of flue gas from increasing over a temperature suitablefor desulfurizing reaction, then it is possible to evaporate watercontained in aqueous ammonia completely in a process vessel.

However, if the sum of water required, the amount of ammonia requiredfor desulfurizing reaction as shown in the above formula (1), and theconcentration of aqueous ammonia are not well-balanced, then gaseousammonia is injected or water is sprayed and injected, separately fromaqueous ammonia. Besides aqueous ammonia, the sprayed and injected waterand/or gaseous ammonia react with SO₂ according to the above formula(1).

In this manner, the reaction product such as ammonium sulfate isconverted into dry powder in the process vessel, and this power iscollected by a by-product collector such as a dry-type electricprecipitator. The powdery by-product collected is ammonium compound suchas ammonium sulfate which can be utilized as a fertilizer. Further, thisdesulfurizing process has such an advantage that no waste water isgenerated differently from a wet-type desulfurizing method in whichsulfur oxides are absorbed by a slurry of lime.

However, in the method for flue gas desulfurization in which aqueousammonia is sprayed and injected into flue gas containing sulfur oxides,and ammonium compound is collected as dry powder, the removal efficiencyof SOx, particularly SO₂ is not generally high. Further, the remainingammonia, in the injected aqueous ammonia, which has not reacted with SOxbecomes gaseous state along with evaporation of water, and thisremaining ammonia is discharged together with the remaining ammonia, ingaseous ammonia injected separately from aqueous ammonia, which has notreacted with SOx, and with the treated flue gas to the atmosphere. Inorder to suppress this ammonia leak, the amount of ammonia to beinjected (the sum of ammonia sprayed and injected as aqueous ammonia andgaseous ammonia injected separately from the aqueous ammonia) isrequired to be reduced, and then the removal efficiency of SOx,particularly that of SO₂ is further lowered. Since unreacted ammoniacorresponding to the lowered removal efficiency is discharged, theamount of ammonia which is leaked is not significantly lowered, althoughthe amount of injected ammonia is reduced.

On the other hand, it is possible to accelerate the desulfurizingreaction by increasing the amount of water to be injected (the sum ofwater sprayed and injected as aqueous ammonia and water sprayed andinjected separately from aqueous ammonia) and lowering the temperatureof flue gas. In this case, the temperature of flue gas is equal toapproximately saturation temperature of water (less than saturationtemperature of water plus 5° C.) in the vicinity of the outlet of theprocess vessel, and therefore water in aqueous ammonia and/or watersprayed separately from aqueous ammonia are difficult to be evaporatedonly by the heat of reaction. Thus, a huge process vessel is required toevaporate water completely, or waste water is generated because water isnot evaporated completely in the process vessel.

Therefore, normally, after aqueous ammonia is sprayed and injected intoflue gas, the mixed gas is irradiated with electron beam in the range ofseveral kGy to over a dozen kGy, and the removal efficiency of SOx,particularly that of SO₂ is improved even in case the temperature (notless than saturation temperature of water plus 5° C.) of flue gas at theoutlet of the process vessel is higher than that of flue gas when it isnot irradiated by electron beam. This is for the following purpose. Theremaining SO₂ which has not been removed in the above formula (1) isoxidized to produce sulfur trioxide (SO₃) or sulfuric acid (H₂SO₄) byradicals such as O, OH, or HO₂ generated from gas molecular such asoxygen or water vapor in the flue gas by irradiation of electron beam,and the generated SO₃ or H₂SO₄ reacts with water (water contained inaqueous ammonia, water vapor evaporated from aqueous ammonia, or watervapor originally contained in flue gas) and ammonia (ammonia dissolvedin aqueous ammonia, gaseous ammonia evaporated from aqueous ammonia, orgaseous ammonia injected separately from aqueous ammonia) to produceammonium sulfate which is recovered according to the following formula(2) and (3).

SO₃+2NH₃+H₂O→(NH₄)₂SO₄  (2)

H₂SO₄+2NH₃→(NH₄)₂SO₄  (3)

However, in order to irradiate flue gas of weight flow Q (kg/s) withelectron beam in absorbed dose of D (kGy), electric power P (kw)calculated by the following formula (4) is consumed.

P(kW)=Q (kg/s)×D(kGy)/(η)(%)/100)   (4)

In the above formula (4), η is the ratio of energy of electron beamabsorbed by the flue gas to the supplied electric power, and this η isnormally in the range of 50 to 80%. Thus, a large amount of electricpower is consumed for desulfurizing flue gas, and therefore it isnecessary to reduce electric power consumption, increase thedesulfurizing efficiency under the condition of the flue gas having ahigh temperature at the outlet of the process vessel (saturationtemperature of water plus 5° C. or more), and reduce ammonia leak.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand an apparatus for flue gas desulfurization which can reduce the costof energy, and lower the amount of ammonia which is leaked whilemaintaining a high desulfurizing efficiency.

In order to achieve the above object, the inventors of the presentapplication have studied intensively and found that the desulfurizingreaction as expressed by the above formula (1) is accelerated by coolingflue gas to a suitable temperature, and spraying aqueous ammonia whichis atomized into fine droplets having a small Sauter mean diameter, andhave accomplished the present invention.

The above object is achieved by the following means.

According to the present invention, there is provided a method for theflue gas desulfurization in which aqueous ammonia is sprayed andinjected into flue gas containing sulfur oxides, and sulfur oxides areconverted into powdery ammonium compound which is collected,characterized in that flue gas is cooled to a temperature range betweena saturation temperature of water and 80° C., and aqueous ammonia issprayed and injected into the cooled flue gas, wherein the aqueousammonia is atomized into droplets having a Sauter mean diameter of 0.5μm to 30 μm for spraying.

According to one aspect of the method of the present invention, theaqueous ammonia is atomized into droplets having a Sauter mean diameterof 0.5 μm to 10 μm.

According to one aspect of the method of the present invention, theaqueous ammonia is atomized into droplets having a Sauter mean diameterof 0.5 μm to 5 μm.

According to one aspect of the method of the present invention, theaqueous ammonia is produced by mixing ammonia gas and water, and theaqueous ammonia is mixed with air, and the mixture is sprayed.

According to one aspect of the method of the present invention, theaqueous ammonia is produced by mixing ammonia gas and air, and thenmixing the mixed gas and water, and ammonia gas in the mixed gas isdissolved partly or wholly in the water to produce aqueous ammonia, andthe remaining mixed gas and the aqueous ammonia are mixed and sprayed.

According to one aspect of the method of the present invention, afterthe aqueous ammonia is sprayed and injected into the flue gas, the fluegas is irradiated with electron beam.

According to the present invention, there is provided an apparatus forflue gas desulfurization in which flue gas containing sulfur oxides isintroduced into a process vessel, aqueous ammonia is sprayed andinjected into the flue gas by an aqueous ammonia spraying apparatus, andsulfur oxides are converted into powdery ammonium compound which iscollected by a by-product collecting apparatus provided downstream ofthe process vessel, characterized in that a gas cooling apparatus isprovided upstream of the process vessel to cool the flue gas to atemperature range between a saturation temperature of water and 80° C.,and the aqueous ammonia spraying apparatus atomizes the aqueous ammoniainto droplets having a Sauter mean of 0.5 μm to 30 μm and sprays theaqueous ammonia.

In other words, there is provided an apparatus for flue gasdesulfurization comprising a process vessel into which flue gascontaining sulfur oxides is introduced, an aqueous ammonia injectingapparatus provided at the inlet of the process vessel for convertingsulfur oxides into powdery ammonium compound by spraying and injectingaqueous ammonia, and a by-product collecting apparatus provideddownstream of the process vessel for collecting powdery ammoniumcompound, characterized in that a gas cooling apparatus is providedupstream of the process vessel for cooling flue gas to a temperaturerange between a saturation temperature of water and 80° C., and theaqueous ammonia injecting apparatus atomizes aqueous ammonia intodroplets having a Sauter mean diameter of 0.5 μm to 30 μm and sprays theaqueous ammonia.

According to one aspect of the apparatus of the present invention, theaqueous ammonia is atomized into droplets having a Sauter means diameterof 0.5 μm to 10 μm and sprayed.

According to one aspect of the apparatus of the present invention, theaqueous ammonia is atomized into droplets having a Sauter mean diameterof 0.5 μm to 5 μm and sprayed.

According to one aspect of the apparatus of the present invention, theaqueous ammonia spraying apparatus comprises an aqueous ammoniagenerating apparatus for mixing ammonia gas and water, and a two-fluidnozzle for mixing the aqueous ammonia and air and spraying the mixture.

According to one aspect of the apparatus of the present invention, theaqueous ammonia spraying apparatus comprises a gas mixer for mixingammonia gas and air, and a two-fluid nozzle for mixing the mixed gas andwater in a gas-liquid mixing compartment and dissolving ammonia gas inthe mixed gas partly or wholly in the water to produce aqueous ammonia,and mixing the remaining mixed gas and the aqueous ammonia and sprayingthe mixture.

According to one aspect of the apparatus of the present invention, theaqueous ammonia spraying apparatus comprises an impact atomizingapparatus for causing the droplets sprayed from the two-fluid nozzle tocollide with an obstacle and further atomizing the droplets.

According to one aspect of the apparatus of the present invention, theaqueous ammonia spraying apparatus comprises an atomizing andclassifying apparatus for selecting droplets having a small diameterfrom the atomized droplets by collision with an obstacle by gravitationand/or the force of the wind and injecting the selected droplets intothe flue gas.

According to one aspect of the apparatus of the present invention, inthe process vessel, after aqueous ammonia is sprayed and injected intothe flue gas, the flue gas is irradiated with electron beam.

The inventors of the present application have found that thedesulfurizing reaction as expressed in the above formula (1) isremarkably accelerated in a gas-liquid interface between aqueous ammoniaand gas, and therefore the removal efficiency of SOx is increased byenlarging the total surface area of Ae (m²/m³) (specific surface area)of droplets of aqueous ammonia per unit volume of gas. Further, in thecase where aqueous ammonia of L (m²) is sprayed and injected into fluegas of G (m³ ), and the droplets of aqueous ammonia are distributed ind-n particle size distribution in which the number of particles havingparticle diameter of d (m) is n, then Ae is calculated by the followingformula (5). $\begin{matrix}{\begin{matrix}{{Ae} = {\left\{ {\Sigma \quad {n4}\quad {\pi \left( {d/2} \right)}^{2}} \right\}/G}} \\{= {\left\{ {\pi \quad \Sigma \quad {nd}^{2} \times \left( {\Sigma \quad {{nd}^{3}/\Sigma}\quad {nd}^{2}} \right)} \right\}/\left\{ {G\left( {\Sigma \quad {{nd}^{3}/\Sigma}\quad {nd}^{2}} \right)} \right\}}} \\{= {{\pi \quad \Sigma \quad {{nd}^{3}/\left( {Gd}_{32} \right)}} = \left\lbrack {{6{\left( {\Sigma \quad {n4}\quad {\pi/3}\left( {d/2} \right)^{3}} \right\rbrack/\left( {Gd}_{32} \right)}} = {6{L/\left( {Gd}_{32} \right)}}} \right.}} \\{= {6{\left( {L/G} \right)/d_{32}}}}\end{matrix}} & (5)\end{matrix}$

Here, d₃₂=Σnd³/Σnd² is called Sauter mean diameter. Therefore, by theabove formula (5), specific surface area of droplets of aqueous ammoniais larger as the Sauter mean diameter is smaller. In this manner, theinventors of the present application have accomplished the presentinvention in which a high desulfurizing efficiency can be achieved byatomizing aqueous ammonia into droplets having a Sauter mean diameter of30 μm or smaller, preferably 10 μm or smaller, and more preferably 5 μmor smaller, and spraying the aqueous ammonia. Gravitation and/or theforce of the wind and injecting the selected droplets into the flue gas.

However, the inventors of the present application have studied furtherand found that if the diameter of droplet of aqueous ammonia isexcessively small when the droplet is sprayed, the desulfurizingefficiency can not be improved. The reason for this is considered asfollows. That is, if the diameter of droplet of aqueous ammonia issmall, evaporation is accelerated due to not only the heat of reactioncaused by the desulfurizing reaction in droplets of aqueous ammonia butalso molecular movement caused by the difference between water vaporpressure in flue gas and water vapor pressure on the surface of droplet.Therefore, if the diameter of droplet is extremely small, water contentis completely evaporated before the desulfurizing reaction as expressedin the above formula (1) proceeds sufficiently on the surface ofdroplet, and hence ammonia in aqueous ammonia is evaporated and thedroplets of aqueous ammonia disappear. Thereafter, SOx, water vapor,gaseous ammonia, and oxygen, if necessary, react only in gaseous phasein which the reaction rate is much slower than that on the surface ofdroplet of aqueous ammonia, with the result that the desulfurizingefficiency is lowered.

In order to solve this problem, the inventors of the present applicationhave made the present invention in which before aqueous ammonia issprayed and injected into flue gas, the flue gas is cooled to atemperature range between a saturation temperature of water and 80° C.,preferably a temperature range between a saturation temperature of waterand 70° C. In this manner, when aqueous ammonia is sprayed and injected,water vapor pressure in flue gas is high, and hence the droplets ofaqueous ammonia are prevented from being evaporated by molecularmovement caused by the difference between water vapor pressure in fluegas and water vapor pressure on the surface of droplet. Thus, aqueousammonia is evaporated mainly due to the heat of reaction caused by thedesulfurizing reaction in droplets of aqueous ammonia, and it ispossible for the droplets to continue to exist until the desulfurizingreaction proceeds sufficiently on the surfaces of droplets. Beforeaqueous ammonia is sprayed and injected into flue gas, even if the fluegas is cooled to approximately a saturation temperature of water, heatother than that used for evaporation of water contained in aqueousammonia in the heat of reaction caused by the desulfurizing reaction isabsorbed in the flue gas, and hence the temperature of flue gas at theoutlet of the process vessel increases to a saturation temperature ofwater plus 5° C. or higher, and it is possible to prevent generation ofwaste water.

Before aqueous ammonia is sprayed and injected into flue gas, even ifthe temperature of flue gas is controlled at a temperature range betweena saturation temperature of water and 80° C., preferably a temperaturerange between a saturation temperature of water and 70° C., when aqueousammonia is atomized into droplets having a Sauter mean diameter of lessthan 0.5 μm, it is very difficult to suppress evaporation of watercaused by molecular movement due to the difference between water vaporpressure in the flue gas and water vapor pressure on the surface ofdroplet. Therefore, it is necessary that the Sauter mean diameter ofdroplets of aqueous ammonia is in the range of 0.5 μm to 30 μm,preferably in the range of 0.5 to 10 μm, and more preferably in therange of 0.5 to 5 μm.

In this manner, the inventors of the present application have found thatafter flue gas is cooled to a temperature range between a saturationtemperature of water and 80° C., preferably a temperature range betweena saturation temperature of water and 70° C., aqueous ammonia is sprayedand injected, and specifically aqueous ammonia is atomized into dropletshaving a Sauter mean diameter of 0.5 μm to 30 μm, preferably 0.5 μm to10 μm, and more preferably 0.5 μm to 5 μm an sprayed, whereby a highdesulfurizing efficiency can be achieved on the condition that thetemperature of flue gas is high at the outlet of the process vessel (asaturation temperature of water plus 5° C. or higher). As describedabove, by enlarging specific surface area of aqueous ammonia containedin gas, the desulfurizing reaction as expressed in the above formula (1)is accelerated, and ammonia is consumed with the removal of SO₂, andhence the amount of remaining ammonia which is leaked can be lowered.

Further, after aqueous ammonia is sprayed and injected, by irradiationof electron beam, the temperature of the flue gas at the outlet of theprocess vessel is further raised, and/or a higher desulfurizingefficiency can be achieved. In this case, flue gas is cooled to atemperature range between a saturation temperature of water and 80° C.,preferably a temperature range between a saturation temperature of waterand 70° C., and then aqueous ammonia is sprayed and injected into thecooled gas. Specifically, aqueous ammonia is atomized into dropletshaving a Sauter mean diameter of 0.5 μm to 30 μm, preferably 0.5 μm to10 μm, and more preferably 0.5 μm to 5μm and sprayed. In this manner,the dose of electron beam becomes smaller than that in the conventionalmethod, and thus electron power consumption can be reduced.

As a method for spraying and injecting aqueous ammonia, there is amethod in which ammonia and water are mixed to produce aqueous ammonia,and the produced aqueous ammonia is mixed with compressed air, and thensprayed. Further, there is a method in which ammonia gas and compressedair are mixed with each other, the mixed gas and water are mixed witheach other, and ammonia gas in the mixed gas is dissolved partly orwholly in water to produce aqueous ammonia, and then the remaining mixedgas and the produced aqueous ammonia are mixed with each other andsprayed. If the amount of aqueous ammonia and water in the sprayedmixture is constant, the concentration of ammonia in aqueous ammonia inthe former method is higher than that in the latter method, and thus thedesulfurizing efficiency becomes high. However, in the former method, alarge-scale apparatus for producing aqueous ammonia by mixing ammoniagas and water is necessary, and it is difficult to change the amount ofammonia and the amount of water to be injected independently accordingto flue gas flow rate and the concentration of sulfur oxides in fluegas. On the other hand, in the latter method, since all amount ofammonia in the mixed gas produced by mixing ammonia gas and compressedair is not necessarily dissolved, the concentration of ammonia inaqueous ammonia tends to be lowered. However, mixing of ammonia gas andcompressed air can be easily conducted, and mixing of the mixed gas andwater and spraying the mixture can be performed by a small-scaletwo-fluid nozzle. Further, the amount of ammonia and the amount of waterto be injected can be independently and easily changed.

Mixing of compressed air and aqueous ammonia, or mixing of the mixed gasof ammonia gas and compressed air, and water, and spraying of them canbe performed by the two-fluid nozzle. Further, the two-fluid nozzle mayincorporate an impact atomizing apparatus for atomizing droplets sprayedby the two-fluid nozzle by impact generated when the droplets collidewith an obstacle, or a particle classifying apparatus for selectingdroplets having a small diameter using gravitation or the force of thewind and injecting the selected droplets into flue gas to make thediameters of the droplets of aqueous ammonia smaller.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 is a schematic view showing a method and an apparatus for fluegas desulfurization according to an embodiment of the present invention;

FIG. 2 is a schematic view showing a method and an apparatus for fluegas desulfurization according to another embodiment of the presentinvention;

FIG. 3 is a schematic view showing a method and an apparatus for fluegas desulfurization according to still another embodiment of the presentinvention;

FIG. 4 is a schematic view showing a method and an apparatus for fluegas desulfurization according to still another embodiment of the presentinvention;

FIG. 5 is a cross-sectional view of a two-fluid nozzle used in theprocess according to an embodiment;

FIG. 6 is a cross-sectional view of the an adapter of the two-fluidnozzle according to an embodiment;

FIG. 7 is a graph showing the relationship among Sauter mean diameter,the desulfurizing efficiency, and absorbed dose of electron beam; and

FIG. 8 is a graph showing the relationship among Sauter mean diameter,the desulfurizing efficiency, and absorbed dose of electron beam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic view showing a method and an apparatus for fluegas desulfurization according to an embodiment of the present invention.

Flue gas containing sulfur oxides generated from a boiler 1 is cooled ina heat exchanger 2, the cooled by a gas cooling apparatus 3 in which theflue gas contacts industrial water, and then introduced into a processvessel 4. On the other hand, ammonia supplied from an ammonia supplyfacility 11 is mixed with industrial water in an aqueous ammoniagenerating apparatus 13 to produce aqueous ammonia, aqueous ammonia ismixed with compressed air in a two-fluid nozzle 12 provided at the inletof the process vessel, and a gas-liquid mixture is sprayed by thetwo-fluid nozzle 12 to inject the atomized droplets of aqueous ammoniainto the flue gas. Thereafter, the produced powdery by-product composedmainly of ammonium sulfate is collected by a dry-tape electricprecipitator 6.

FIG. 2 is a schematic view showing a method and an apparatus for fluegas desulfurization according to another embodiment of the presentinvention.

Flue gas containing sulfur oxides generated from a boiler 1 is cooled ina heat exchanger 2, then cooled by a gas cooling apparatus 3 in whichthe flue gas contacts industrial water, and then introduced into aprocess vessel 4. On the other hand, ammonia supplied from an ammoniasupply facility 11 is mixed with compressed air in a line mixer 14 toproduce a mixed gas, and the mixed gas and industrial water are mixedwith each other in a gas-liquid mixing compartment of a two-fluid nozzle12 provided at the inlet of the process vessel. Thus, ammonia gas in themixed gas is dissolved partly or wholly in water to produce aqueousammonia, and the remaining mixed gas and the produced aqueous ammoniaare mixed in the two-fluid nozzle 12 and sprayed, whereby atomizeddroplets of aqueous ammonia and the remaining gaseous ammonia areinjected into the flue gas. Thereafter, the produced powdery by-productcomposed mainly of ammonium sulfate is collected by a dry-type electricprecipitator 6. The line mixer 14 constitutes a gas mixer.

FIG. 3 is a schematic view showing a method and an apparatus for fluegas desulfurization according to still another embodiment of the presentinvention.

Flue gas containing sulfur oxides generated from a boiler 1 is cooled ina heat exchanger 2, then cooled by a gas cooling apparatus 3 in whichthe flue gas contacts industrial water, and then introduced into aprocess vessel 4. On the other hand, ammonia supplied from an ammoniasupply facility 11 is mixed with industrial water in an aqueous ammoniagenerating apparatus 13 to produce aqueous ammonia, and aqueous ammoniais mixed with compressed air in a two-fluid nozzle 12 provided at theinlet of the process vessel, and a gas-liquid mixture is sprayed by thetwo-fluid nozzle 12 to inject atomized droplets of aqueous ammonia intothe flue gas. The flue gas into which aqueous ammonia has been sprayedand injected is irradiated with electron beam by an electron accelerator5 in the process vessel 4. Thereafter, the produced powdery by-productcomposed mainly of ammonium sulfate is collected by a dry-type electricprecipitator 6.

FIG. 4 is a schematic view showing a method and an apparatus for fluegas desulfurization according to still another embodiment of the presentinvention.

Flue gas containing sulfur oxides generated from a boiler 1 is cooled ina heat exchanger 2, then cooled by a gas cooling apparatus 3 in whichthe flue gas contacts industrial water, and then introduced into aprocess vessel 4. On the other hand, ammonia supplied from an ammoniasupply facility 11 is mixed with compressed air in a line mixer 14 toproduce a mixed gas, and the mixed gas and industrial water are mixedwith each other in a gas-liquid mixing compartment of a two-fluid nozzle12 provided at the inlet of the process vessel. Thus, ammonia gas in themixed gas is dissolved partly or wholly in water to produce aqueousammonia, and the remaining mixed gas and the produced aqueous ammoniaare mixed in the two-fluid nozzle 12 and sprayed, whereby atomizeddroplets of aqueous ammonia and the remaining gaseous ammonia areinjected into the flue gas. The flue gas into which aqueous ammonia andgaseous ammonia have been sprayed and injected is irradiated withelectron beam by an electron accelerator 5 in the process vessel 4.Thereafter, the produced powdery by-product composed mainly of ammoniumsulfate is collected by a dry-type electric precipitator 6. The linemixer 14 constitutes a gas mixer 4.

FIG. 5 shows the two-fluid nozzle used in the process shown in FIGS. 1through 4 according to an embodiment. Further, FIG. 6 shows an adapterof the two-fluid nozzle according to an embodiment. A mixed gas ofammonia and compressed air, and water, or compressed air and aqueousammonia supplied from a gas-liquid double tube 21 are mixed in agas-liquid mixing compartment 22 of the two-fluid nozzle, and sprayedfrom a forward end of a nozzle chip 23.

The experiments using the apparatuses shown in FIGS. 2 and 4 willdescribed below. However, the present invention is not limited to theseexperiments.

Flue gas of 1,500 m³N/h containing 850 ppm of sulfur oxides andgenerated by the boiler 1 was cooled to 150° C. by the heat exchanger 2,and then cooled to a saturation temperature of water plus 10° C., i.e.50° C. in the gas cooling apparatus 3 in which the flue gas contactsindustrial water. Thereafter, the flue gas was introduced into theprocess vessel 4. On the other hand, ammonia of 2.3 m³N/h supplied fromthe ammonia supply facility 11 was mixed with compressed air in the linemixer 14 to produce a mixed gas. The mixed gas and industrial water of18 kg/h were mixed with each other in the gas-liquid mixing compartmentin the two-fluid nozzle 12 provided at the inlet of the process vessel,and ammonia gas in the mixed gas was dissolved partly or wholly in waterto produce aqueous ammonia. Thereafter, the remaining mixed gas and theproduced aqueous ammonia were mixed with each other in the two-fluidnozzle and sprayed, and hence the atomized droplets of aqueous ammoniaand the remaining gaseous ammonia were injected into the flue gas.Further, in the experiments using the apparatus shown in FIG. 4, theflue gas into which aqueous ammonia and gaseous ammonia were sprayed andinjected was irradiated with electron beam by an electron accelerator 5in the process vessel 4. In either case, the produced powdery by-productcomposed mainly of ammonium sulfate was collected by the dry-typeelectric precipitator 6.

In the experiments, the type of the two-fluid nozzle, the pressure ofcompressed air, and the flow rate of compressed air were changed, andhence the Sauter mean diameter (d₃₂=Σnd³/Σnd²) of droplets of aqueousammonia sprayed and injected was changed. Further, in the experimentsusing the apparatus shown in FIG. 4, beam current in electron beam waschanged and absorbed dose was changed to examine the change of thedesulfurizing efficiency.

FIG. 7 shows the results of the experiments. As shown in FIG. 7, thesmaller the Sauter mean diameter of droplets of aqueous ammonia is, thehigher the desulfurizing efficiency is. Particularly, even if absorbeddose is zero in the experiments using the apparatus shown in FIG. 2,when the Sauter mean diameter is about 10 μm, a high desulfurizingefficiency of approximately 50% can be achieved. As a method forremoving sulfur oxides, this is practically desirable value. Further,FIG. 7 shows that even if absorbed dose is zero, when the Sauter meandiameter is about 30 μm, a relatively high desulfurizing efficiency ofabout 40% can be achieved. As a method for removing sulfur oxides, thisis practically usable value. Further, in the case where the Sauter meandiameter is more than 30 μm, when it is 100 μm or lower, even if theabsorbed dose is zero, a desulfurizing efficiency of 20% or higher canbe achieved. However, in the case where the Sauter mean diameter is morethan 30 μm, water content in sprayed aqueous ammonia is not evaporatedcompletely, and waste water is generated in the process vessel 4 and theduct connecting the process vessel 4 and the dry-type electricprecipitator 6. Therefore, it is desirable that the Sauter mean diameteris equal to or smaller than 30 μm.

Instead of the two-fluid nozzle 12 shown in FIGS. 2 and 4, there wasprovided an aqueous ammonia spraying apparatus which incorporated animpact atomizing apparatus with an obstacle and a atomizing andclassifying apparatus which were added to the two-fluid nozzle, wherebydroplets sprayed by the two-fluid nozzle collided with the obstacle tobe further atomized, the droplets having a small diameter were selectedfrom the atomized droplets by using gravitation and the force of thewind, and the selected droplets were sprayed. Thus, the followingexperiments were conducted in the same manner as FIG. 7. FIG. 8 showsthe results of the experiments. As shown in FIG. 8, when the Sauter meandiameter of droplets of aqueous ammonia is about 5μm, a highdesulfurizing efficiency of about 60% can be achieved. As a method forremoving sulfur oxides, this is practically desirable value. Further, asshown in FIG. 8, up to the Sauter mean diameter of 0.5 μm, the smallerthe diameter of droplet is, the higher the desulfurizing efficiency is.However, in the case where the Sauter mean diameter is smaller than 0.5μm, conversely the desulfurizing efficiency is lowered. Therefore, it isdesirable that the Sauter mean diameter is equal to or larger than 0.5μm.

According to a method and an apparatus for flue gas desulfurization inthe present invention, flue gas is cooled to a saturation temperature ofwater or higher and 80% or lower, and aqueous ammonia is sprayed andinjected into the flue gas. Specifically, aqueous ammonia is atomizedinto droplets having a Sauter mean diameter of 0.5 μm, to 30 μm,preferably 0.5 μm to 10 μm, and more preferably 0.5 μm to 5 μm. Thus,energy cost is reduced, and the amount of ammonia which is leaked islowered while maintaining a high desulfurizing efficiency.

Further, after aqueous ammonia is sprayed and injected into flue gas, byirradiation of electron beam, the temperature of flue gas at the outletof the process vessel is further raised and/or a higher desulfurizingefficiency can be achieved.

The present invention is applicable to a flue gas treatment system forremoving sulfur oxides contained in combustion flue gas generated whenvarious fuel such as coal or oil is combusted.

What is claimed is:
 1. A method for flue gas desulfurization in whichsulfur oxides in flue gas are converted into powdery ammonium compound,comprising the steps of: providing an aqueous ammonia spraying deviceconfigured to atomize aqueous ammonia into droplets; cooling flue gascontaining sulfur oxides down to a temperature between a saturationtemperature of water and 80° C.; adjusting the aqueous ammonia sprayingdevice such that the aqueous ammonia is atomized into droplets having aSauter mean diameter of 0.5 μm to 30 μm; and spraying said aqueousammonia into the flue gas which has been cooled in the cooling step. 2.A method according to claim 1, wherein said adjusting step comprisesadjusting the aqueous ammonia spraying device such that said aqueousammonia is atomized into droplets having a Sauter mean diameter of 0.5μm to 10 μm.
 3. A method according to claim 1, wherein said adjustingstep comprises adjusting the aqueous ammonia spraying device such thatsaid aqueous ammonia is atomized into droplets having a Sauter meandiameter of 0.5 μm to 5 μm.
 4. A method according to claim 1, whereinsaid adjusting step comprises mixing ammonia gas and water to producethe aqueous ammonia, adjusting pressure and flow rate of air and mixingsaid aqueous ammonia with the air.
 5. A method according to claim 1,wherein; said aqueous ammonia is produced by mixing ammonia gas and air,and then mixing with water to dissolve said ammonia gas at least partlyin said water; and said atomizing step comprises mixing an undissolvedportion of said ammonia gas and air with said aqueous ammonia.
 6. Amethod according to claim 1, further comprising irradiating said fluegas sprayed with said aqueous ammonia with electron beam.